Internal bubble cooling control system and method

ABSTRACT

An internal bubble cooling air control system in a blown film apparatus has sensors arranged to increase response time and reduce interference, and includes a high speed valve for fast actuation of air flow from a controller in response to signals received from the sensors.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an internal bubble cooling (IBC)air control for a plastic blown film apparatus.

[0002] When blown film is extruded, it typically is in the form of acontinuous, vertically oriented tube. The tube, which is in a moltenstate as it exits a die, expands in diameter as it is pulledcontinuously upward. The diameter stabilizes to a more or less constantvalue when the tube cools sufficiently to solidify a short distance fromthe die at what is called the frost line. Air cooling systems such asexternal air rings and IBC systems within the tube are provided close tothe exit of the die to ensure that the tube cools quickly enough toremain stable.

[0003] The tube usually passes through a bubble cage, which minimizesunwanted tube motion and also determines the final tube size if the cageis allowed to contact the tube while the tube is still molten. Aftersolidifying, the tube passes through a flattening device, known as acollapsing frame, to convert the inflated tube into a flattened out filmwith no air inside. This film is pressed together by motorized nip rollsthat continually draw the film upward and away from the extrusionprocess to form what is call “layflat.” The die and nip roll act asseals, which in steady state, form a trapped, column of air withconstant volume inside the tube.

[0004] Film processors employing IBC systems realize production rategains on the order of 20% to 50%. In known IBC systems, such as thatdescribed in U.S. Pat. No. 4,243,363, air passages are provided throughthe die to allow for significant air flow into and out of the tube. Airsupply and exhaust systems act under the supervision of a control systemin response to measured tube size. The control system adjusts the flowof air to be in balance so that a constant, desired tube size ismaintained.

[0005] For IBC systems to remain stable, there cannot be a significantclosed loop lag time between the time when an air flow change firstoccurs and when the new tube size actually gets sensed by thecontroller. Excessive total lag time causes a tube to oscillate in size.Typical oscillation periods induced by IBC systems are generally 4 to 6seconds in duration. This implies that the closed loop lag time mustremain less than about 1 second to 1.5 seconds or else the lag time willbe greater than 90 degrees out of phase and oscillation will result.Present art IBC systems have a hardware sensor response time andactuation of corrective air flow lag time of about ½ to 1 second. Totalclosed loop lag time includes this hardware lag time and an additionalprocess related sensing lag time caused by size changes taking place ata point prior to where sensing occurs.

[0006] Bubble instability prevents film processors from using IBCsystems to achieve higher production rates when extruding many of thenewer high performance materials. This instability is caused by aprocess related sensing lag time that is great enough to force the IBCsystem into oscillation. Sensing lag time is the time it takes for themolten region of the bubble, which reacts in size to the influence ofIBC air flow changes, to move along the process until it has solidifiedinto a final dimension that can be accurately measured at or just afterthe frost line. Traditionally, older resins such as Low DensityPolyethylene (LDPE) react in size to air flow changes very close to orat the frost line, thus providing for minimal sensing time lag time andmaking it easy to control bubble stability. In the early 1980's,however, Linear Low Density Polyethylene (LLDPE) became commerciallyviable. LLDPE reacts just prior to reaching the frost line causing aslightly longer sensing time lag of ½ to 1 second. Processors foundLLDPE more difficult to control, but by blending in small amounts ofLDPE and/or by lowering the IBC size sensors to the first line orslightly below the frost line, bubble stability could be maintained.Lowering the sensors this far, however, has a disadvantage in that themeasured size is no longer accurate. This accuracy problem has beenpartially addressed by control systems providing for easy re-calibrationof measured tube size. More recently, new materials such as metalloceneshave further lowered the reaction point, making them difficult orimpossible to control with IBC systems. It would be advantageous tosense directly at a reaction point that is well below the frost linewithout adverse effects on measured size.

[0007] An additional problem arises due to the sensitivity of sensorpositioning in that the frost line does not stay in one place over time.As material and ambient conditions vary during production, the locationof the frost line can change by several inches. This movement causes theprocessor to constantly monitor and adjust sensor positioning to trackwith the frost line. Presently, sensor adjustments are made manually bythe operator, usually in response to tube oscillation that suddenlyappears or actual tube size changes that occur due to degraded tube sizecalibrations. It would be advantageous to automatically reposition IBCsensors relative to the frost line to maintain sensing lag timeconstant, thus preventing the onset of tube oscillations. Automaticpositioning would also serve to minimize the need for tube sizere-calibration although bubble shape effects that can accompany changesin the location of the frost line might still warrant re-calibration,but significantly less often.

[0008] Another problem relates to a well documented characteristic thattubes naturally vary in size over short periods of time, independent ofany IBC volumetrically related instability, just as processes not usingIBC systems do. Experimentation has revealed that with materials in usetoday, tube size naturally changes in a periodic manner with a frequencyof about 1 to 2 Hertz. Tube size changes by as much as ⅓ to ½ inch oflayflat for processes with light or no contact with the bubble cage andfrom {fraction (1/10)} to ⅓ inch for processes that use the bubble cageto squeeze in just below the frost line and size the bubble. It is adisadvantage to squeeze the tube since marks and scratches routinelyresult from contact points with the bubble cage. Without squeezing witha cage, IBC control systems must have a total system response time(sensing lag time+sensing response time+actuator response time) of about0.1 seconds (10 hertz) or better to control these natural fluctuations.Presently, sensing lag time, response time, and actuator response timeindividually are each too great to allow for control of natural tubesize changes so each must be addressed. It would be beneficial if totalsystem lag time and accuracy could be brought to a level where higherfrequency natural bubble instabilities could be controlled by IBCcontrol systems without reducing film quality due to scratches.

[0009] IBC control systems employ mechanical, optical, and acousticsensors for monitoring tube size. Mechanical sensors cause marks on theresulting film and optical sensors tend to get dirty and unreliable inthe typical blown film plant environment making them unsuitable for manyapplications in blown film production. Acoustic sensors are preferredbecause they provide non-contact sensing and are very reliable in aplant environment. Such systems, however, do have slow sampling rate andproblems with sensor interference when more than one acoustic sensor isplaced into service around a tube. Acoustic sensors operate by sendingout a conical ultrasonic sound pulse and measuring the time it takes forthe pulse to bounce off a target, such as the tube, and return back tothe sensor which sent the pulse. Distance is then calculated bymultiplying the time of flight by the speed of sound in the ambient airthat the pulse just traveled through. Blown film bubbles tend to flutterand move around, causing the sound pulses to bounce in many differentdirections. If the pulse passes by a sensor other than the one that sentit, interference can and usually does occur. Additionally, anoriginating sensor may not receive the return signal, resulting in amissed target response. Yet another problem is that intervening objectssuch as operating personnel servicing the system can prevent theacoustic sensor from detecting the bubble as a target, and instead suchpersonnel become the target. These errors lead to incorrect reaction bythe control system and thus cause instability in the tube size. Presentart systems employ various techniques to minimize these problems at theexpense of response time.

[0010] A common method and the least expensive overall acousticalapproach is to use only one sensor as described in U.S. Pat. No.5,104,593. This approach suffers the most from inaccuracy due to tubemotion. The swaying motions and the flutter of the tube common to theblown film process is perceived by the sensor to be a change in sizewith corresponding degradation in performance. To combat this inaccuracyand achieve good size control, single sensor systems typically requirethe use of a bubble cage to surround the tube and forcefully determinetube size, thereby causing scratches and marks in the finished film.Interference with other sensors is not an issue, and this approachallows for sampling rates of 25 to 30 times per second, but a dual stagesoftware filtering system is required to prevent misidentifying noise orbubble sway as an actual change in tube size thus allowing it to trackonly relatively gradual changes in tube size. The first stage of thefilter requires an average of 8.5 samples to effect a change in outputyielding an approximate ⅓ second response time. The second stage furtherlimits response time.

[0011] Another common method, as described in U.S. Pat. No. 4,377,540,is to use more than one acoustic sensor by alternately operating eachsensor one at a time in sequence and wait long enough between samples toprevent interference. In this approach multiple sensors sample tube sizepreferably from diametrically opposed positions, thus canceling theeffects of tube sway. Due to the time delays present and lower operatingfrequencies, however, these systems allow for only 10 samples to betaken per second. True diameter measurements without influences fromswaying require at least 2 samples limiting this approach to ⅕ secondsensing response which is then further limited by filtering elementsnecessary for outside noise immunity.

[0012] Yet another approach uses multiple sensors operating in a freerun mode with sampling rates of 40 to 50 times per second without regardfor alternating sensor operation. Here, sensors are placed so that straysignals typically bounce away from one another. Interference can stilloccur, however, so a special rate filter is employed to eliminate theeffects of inter-sensor interference and missed targets. Experimentshave determined that this approach has a typical sensor response time onthe order of ⅕ to ⅛ second.

[0013] None of the present systems can tolerate the accidental placementof intervening objects in front of acoustic sensors. Typically, objectsplaced in front of sensors lead to significant bubble instabilitysufficient to force the extrusion line to shut itself down.

[0014] Significant limitations also exist with actuators that adjust airflow. Most IBC air flow actuators are butterfly style valves. Thesevalves suffer from inaccuracy due to linkage backlash and are eithermotorized or respond to the position of a spring loaded air cylinder.Other actuators are of the bladder valve design, which has no backlash,and operates by inflating or deflating a series of bladders containedinside the air system piping to change the resulting air flowrestriction. Yet another design uses a spring loaded air cylinder toposition a linear slide valve that also has no backlash, but suffersfrom problems with positioning errors due to drag in the valve and aircylinder. Experiments have revealed that motorized valves have reactiontimes of about ½ to 1 second, while spring loaded air cylinders andbladders use pneumatic systems that move air to pressurize ordepressurize the actuator with total reaction time of ⅓ to ¾ of asecond. Unfortunately, actuators generally in use today in blown filmsystems do not have the reaction time or accuracy required forcontrolling natural high frequency bubble instability.

SUMMARY OF THE INVENTION

[0015] The present invention includes an internal bubble cooling (IBC)and control system using acoustic sensors that measure tube sizeresulting from the blown film extrusion apparatus. The IBC controlsystem provides for size sensors located above the frost line where thesize of the bubble is stable, to maintain tight calibration of actualtube size. Separate control sensors are adjustable in position at avertical location below that of the size sensors. These control sensorsare preferably located at or below a point just above the frost line,and may be well below the frost line, so that in production thesecontrol sensors can be positioned at the point of maximum bubble sizereaction to internal air flow changes to compensate for high speed sizefluctuations.

[0016] Size and control sensors initially are calibrated by havingoperating personnel inputting actual manually measured size into thesystem and applying this size data independently to each sensor toestablish a separate respective calibration value. Size sensorcalibrations remain fixed until a next operator calibration. These sizesensors are then used in an integrating mode to constantly re-calibrateeach separate control sensor, thus allowing them to be located wherevernecessary just above or below the frost line to control the process.

[0017] The initial calibrations for control sensors are storedseparately and are compared to the integrated re-calibration ongoing foreach control sensor. As the position of the frost line naturally changesover time, the control sensor location is automatically adjusted,usually by means of positioning the sizing cage to which they areattached. Position adjustments are made until the integratedre-calibration again matches the initial calibration for the controlsensors.

[0018] Signals representing a position of the bubble are then providedto control logic in a controller to cause more or less cooling air toflow onto the bubble.

[0019] The present invention also includes a sensing method thatrequires no time averaging of signals to eliminate bad readings andallows for full speed operation of the sensors. Preferably, more thanone sensor is used for sizing and more than one is used for controlling.Use of multiple sensors provides a redundancy that allows for rapidfiltering by analyzing each sensor's response for false readings.Statistically, there will be at least one sensor that detects the bubbleclose to where it has been within a tolerance of preferably 1 to 2inches from previous measurements. All sensors are compared to oneanother, and any sensor that falls outside a specified tolerance bandare ignored. Further, if a majority of the responses from a given sensorare bad, that sensor is automatically taken out of service withoutshutting down the process. A warning, such as a warning light, can beturned on (or a normally “on” light turned off) separately for eachsensor to inform the operator that a sensor is being automaticallyignored; if a sensor is being ignored, the warning light remains onpermanently until the sensor begins to again provide a majority of goodresponses.

[0020] The present invention further includes a control system whichsynchronizes and simultaneously fires all acoustic sensors, and thenwaits a delay time, such as 3 to 16 milliseconds (depending on sensorarrangement), that is long enough for any stray signals to bounceharmlessly away without causing inter-sensor interference beforerepeating the sequence. By desirably positioning size and controlsensors each as pairs of diametrically opposed sensors, true bubble sizemeasurements can be made within a single multiple-sensor cycle withoutthe unwanted effects of bubble sway and without interference problems.Combining synchronized rapid firing with redundancy filtering allows forsimultaneous, reliable and accurate measurement of the tube for controland sizing between 60 and 300, and preferably 100, times per second,with no need for further filtering.

[0021] Yet another feature of the present invention relates to a linearvalve which both precisely meters the amount of air flow and actuates ata very high speed. The linear valve operates by positioning a pistoninside of a double pipe arranged with longitudinal slots that, whenpartially uncovered, control the amount of air flowing from theinnermost to the outermost pipe. According to the invention, a linearservo motor is employed (rather than an air cylinder) for pistonpositioning together with a vertical orientation, and the piston andwall around the piston are designed for minimal friction to yield fastand precise metering of air flow.

[0022] Other features and advantages will become apparent from thefollowing detailed description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic side view of an extrusion line and aninternal bubble cooling and control system according to the presentinvention. FIGS. 2(a)-2(c) are graphic depictions of the operation ofthe filtering system according to the present invention, showingautomatic sensor shut-down and turn-on.

[0024]FIG. 3 is a graphic depiction of the actual operation of thefiltering system on a blown film extrusion line for two sensorsdiametrically opposed with the resulting tube sway, tube size, andflutter filter results.

[0025]FIG. 4 is a plan view of a sensor arrangement for known systems.

[0026]FIGS. 5 and 6 are plan views of the sensor arrangement for sizeand control sensors respectively according to the present invention.

[0027]FIG. 7 is a graphic depiction of the operation of an acousticsensor with an interfering sensor source triggered simultaneously and athigh repetition rates according to the present invention.

[0028]FIG. 8 is a cross-section view of a servo driven linear airactuator according to the present invention.

DETAILED DESCRIPTION

[0029] Referring to FIG. 1, plastic resin is provided into an extruder12 from a holding bin 10. Extruder 12 provides a plastic melt to thebottom of a blown film die 14, which in turn provides an annular plasticmelt concentric with a process centerline 20. The melt passes through acooling ring 16, which blows external cooling air 17 onto the annularplastic melt. The melt forms a conically expanding molten tube 18, whichsolidifies into a continuous cylindrical bubble 24 above a freeze line22. Bubble 24 may be stabilized by a bubble cage 25. Bubble 24 isconverted to a layflat sheet of film 34 as it passes through collapsingshields 26 and 28, and motorized nip rolls 30 and 32 that continuallydraw the film upwardly from the extrusion process. Bubble 24 is thusconverted into a finished plastic film product, such as plastic bags orrolls of film.

[0030] The finished size of film 34 is directly related to the diameterof bubble 24 according to the following formula:

(layflat sheet of film 34 width)=pi*(diameter of bubble 24)/2

[0031] The diameter of bubble 24 is determined by bubble volume 36contained on all sides by bubble 24, conically expanding molten tube 18,the top surface of die 14, and the nip point of motorized nip rolls 30and 32. Air is either trapped inside bubble volume 36 or is continuouslycirculated into and out of bubble volume 36 by supply blower 38 andexhaust blower 40, respectively. Outgoing air flow 104 from supplyblower 38 is directed into bubble volume 36 though passages in die 14 sothat internal bubble cooling (IBC) air 42 may be used to more quicklycool conically expanding molten tube 18 and thereby increase totalsystem throughput and is balanced with exhaust air 41 that is directedout of bubble volume 36 though passages in die 14 so as to maintain aconstant size bubble 24.

[0032] The total amount of air passing into bubble volume 36 throughsupply blower 38 must equal the total amount of air exiting throughexhaust blower 40, or else a size change will occur immediatelysomewhere (depending on material types) within the conically expandingmolten tube 18. Due to the upward motion of bubble 24 caused by theaction of motorized nip rolls 30 and 32, this size change willaccelerate from typically slow speeds just above die 14 through theconically expanding molten tube 18 region and ultimately translateupwards in bubble 24 at the speed of the motorized nip rolls 30 and 32,and eventually appearing as a size change in film 34.

[0033] In order to control accurately the finished size of film 34, IBCcontroller 44 is used to balance air flow into and out of bubble volume36 by altering the speeds of supply blower 38 and exhaust blower 40.Additionally, to obtain finer size control of film 34, optional air flowvalve 46 rapidly modulates incoming air flow 100 from supply blower 38as outgoing air flow 104. Air flow control valve 34 can be located inthe exhaust air piping system; for cleanliness reasons, however, it ispreferably located in the supply air piping system to minimize fouling.In systems not utilizing air flow valve 46, incoming air flow 100 fromsupply blower 38 and outgoing air flow 104 are the same air flow streamsand are adjusted by means of speed changes to supply blower 38.

[0034] IBC control system 44 utilizes a closed loop control strategy fordetermining what change to make in the balance of air flow by measuringthe final dimension of bubble 24 somewhere at or above frost line 22using sensors 48 a and 48 b. Although sensors 48 a and 48 b can be ofany type, it is preferred that they be non-contacting sensors, andpreferably acoustic sensors, so as not to contact bubble 24 and hencepotentially mark it. One or more sensors can be used, but it ispreferred to employ a pair of sensors located on opposite sides ofbubble 24 to compensate for swaying motion exhibited by bubble 24 andthat is not prevented by bubble cage 25. Compensating sensor 50 isprovided to correct for influences on acoustic sensors 48 a and 48 b byambient air conditions such as temperature, humidity, and pressure,changes in which can change the speed of sound.

[0035] The time it takes for size changes in bubble 24 to translateupwardly from within conically expanding molten tube 18 to the sensinglocation of sensors 48 a and 48 b can be quite lengthy. If this timegrows too large (greater than about 1 to 2 seconds), control loopinstability will result. Newer plastic materials now react in a regionsignificantly below frost line 22 and thus do not allow sufficient timefor size changes to even reach frost line 22 in under the required 1 to2 seconds. Sensors 52 a and 52 b are therefore provided below frost line22 and are aimed directly at a reaction point where size changesactually occur in conically expanding molten tube 18, in order to reducethe lag time from the reaction point to the sensing point of sensors 52a and 52 b to near or equal to zero. IBC controller 44 uses this rapidresponse of sensors 52 a and 52 b to accurately control the size of thereaction point within conically expanding molten tube 18.

[0036] Rather than measuring the true diameter of the reaction point,IBC controller 44 continuously calibrates sensors 52 a and 52 b to thefinal size of bubble 24 using an internal integrating loop dependent onmeasurements taken by sensors 48 a and 48 b. Calibration of sensors 48 aand 48 b are independently done by an initial direct input to controller44 of measurements take manually by operating personnel 54. Controller44 stores these calibration and continuous calibration values separatelyfor each sensor 48 a, 48 b, 52 a, and 52 b, respectively. When theseseparate calibration values are added to actual sensor distancesmeasured by sensors 48 a, 48 b, 52 a, and 52 b, respectively, eachresults in a value equal to the actual size of bubble 24. This approachallows for any sensor to be taken temporarily or permanently out ofservice without impacting the measured size of bubble 24 and thereforewithout causing the process to shut down. When any sensor 48 a, 48 b, 52a and/or 52 b is taken offline for extended periods of time (severalseconds or more), control performance may be temporarily diminished dueto the sway which may be present in bubble 24, but this situation ispreferable to shutting down the process. Periods of short duration donot significantly affect the control.

[0037] Initial calibration of sensors 52 a and 52 b are alsoindependently done by an initial direct input to controller 44 and arestored separately and are compared to the continuous calibration valuesbeing stored for sensors 52 a and 52 b. As the position of the frostline 22 naturally changes over time, the sensors 52 a and 52 b areautomatically re-positioned, usually by means of re-positioning bubblecage 25 to which they are attached. Position adjustments are made untilthe continuous calibration values again match the initial calibrationvalues stored for sensors 52 a and 52 b. The monitoring and detection ofthe frost line is generally known, e.g., through the use of an infraredsensor.

[0038] Referring also to FIGS. 2 and 3, controller 44 continuouslymonitors and redundantly compares sensors 48 a, 48 b, 52 a and 52 b to acurrent size 74 of bubble 24 for errors. An allowed deviation bandbounded by 72 a and 72 b (FIG. 2a) of about twice the normal deviationpresent for non-error samples (about 1 inch) is applied to a currentsize 74 and compared to the most recent samples 76 each for sensors 48a, 48 b, 52 a and 52 b. Any error samples 56 a, 56 b, 56 c, 56 d and 56e which fall outside the prescribed deviation band 72 a and 72 b causethe most recent good sample 58 a, 58 b, 58 c, 58 d and 58 e fromrespective sensor 48 a, 48 b, 52 a and 52 b to be used as a lock-in forsample 60 a, 60 b, 60 c, 60 d and 60 e in place of error sample 56 a, 56b, 56 c, 56 d and 56 e respectively. Current size 74 is thenrecalculated as either the mean (preferred), median or mode of the mostrecent samples 76 or lock-in samples taken from sensors 48 a, 48 b, 52 aand 52 b. This approach prevents large transients due to such errorsamples from inadvertently affecting the control of the size of bubble24.

[0039] Additionally, operating personnel 54 (FIG. 1) routinely need toaccess the area around conically expanding molten tube 18 to monitor theperformance of the process and to make optimizing changes in suchequipment as cooling ring 16. Occasionally, operating personnel 54inadvertently place hands or other extremities in front of sensors 48 a,48 b, 52 a and/or 52 b causing errors in the measured distance to bubble24 or conically expanding molten tube 18. Other errors occur due toflutter and swaying motion of bubble 24 or conically expanding moltentube 18 when acoustic sensor signals bounce off at angles away from theoriginating sensor 48 a, 48 b, 52 a or 52 b such that the sensor 48 a,48 b, 52 a, or 52 b does not detect the target and assumes a target atits respective maximum range. In error situations such as these, manysuccessive error samples 56 a and 56 b will typically result.

[0040] Controller 44 uses the most recent good samples 58 a and 58 b toact as lock-in samples 60 a and 60 b for each successive error sample 56a and 56 b, respectively, until controller 44 determines that a simplemajority of samples results are due to an error condition. If such anerror condition is detected controller 44 shuts off the respectivesensor 48 a, 48 b, 52 a, and 52 b until a simple majority of sampleresults are good. A simple majority of samples is preferably determinedby adjusting an error/good count 62 upward by one count for an errorsignal and downward by 1 count for a good signal. If the error/goodcount 62 reaches a predetermined maximum count 64 (e.g., a count of 16),the error/good count 62 is prevented from going above the maximum count64 and the sensor 48 a, 48 b, 52 a and 52 b shuts down at sampleposition 68. If the error/good count reaches zero count 66, the count isprevented from dropping below zero count 66 and sensor 48 a, 48 b, 52 a,or 52 b turns back on at sample position 70. This process provides alsofor a lag time prior to turning on or off a good or bad sensor 48 a, 48b, 52 a and 52 b.

[0041] While a sensor is not providing good samples, a notification isprovided, such as a flight changing from on to off (or vice versa).

[0042]FIG. 4 illustrates the known use of sensors 53 a, 53 b, 53 c, and53 d, and compensating sensor 51 all at one height. For proper responseto occur, sound must be issued by each sensor 53 a, 53 b, 53 c, 53 d and51 and follow the path 77 a, 77 b, 77 c, 77 d and 83 straight out andback to sensor 53 a, 53 b, 53 c, 53 d and 51, respectively. In thisconfiguration, possible interference paths 79 a, 79 b, 79 c, and 79 dbetween adjacent sensors occur. Sensor signals originating from sensor53 a can reflect off of bubble 24 and false trigger sensor 53 d or 53 balong paths 79 a or 79 b, respectively, or vice versa, and similarly foreach of adjacent sensors 53 a, 53 b, 53 c and 53 d around bubble 24.Additionally, sensors 53 c and 53 b can interfere or be interfered bysensor 51 along paths 81 a and 81 b respectively.

[0043]FIG. 5 and 6 illustrate the use of sensors 48 a, 48 b, 52 a, 52 b,and compensating sensor 50 at differing heights to minimize possibleinterference paths according to an embodiment of the present invention.Sensors 48 a, 48 b and compensating sensor 50 are maintained above frostline 22 in a preferentially diametrically opposed position and farenough separated from the plane of sensors 52 a and 52 b mounted near toor below frost line 22 so as to eliminate possible interference pathsbetween adjacent sensors 48 a, 48 b, 52 a, and 52 b as shown in FIG. 4.Only one path 80 b for interference between sensor 48 b and compensatingsensor 50 exists.

[0044]FIG. 7 graphically depicts a method for eliminating thepossibility for inter-sensor interference by simultaneously transmittingsound pulses 84 respectively from each of sensors 48 a, 48 b, 52 a, 52 band compensating sensor 50. The length of proper measurement paths 76 a,76 b, 76 c, 76 d, and 82 are the shortest compared to interference paths78 a, 78 b, 78 c, 78 d, 80 a, and 80 b, and therefore the first returnsignal 86 which can be received by sensors 48 a, 48 b, 52 a, 52 b, andcompensating sensor 50 are the ones which originated from themselves.Interfering stray return signals 88 coming from other sensors 48 a, 48b, 52 a, 52 b, and compensating sensor 50 will only arrive afterward.Acoustic sensors 48 a, 48 b, 52 a, 52 b, and compensating sensor 50 willrespond to the first return signal 86 or stray return signal 88 thatthey hear, and thus interference will not occur.

[0045] Typical processes make layflat film 34 that is 100 inches (about250 cm) or less across. Sensors 48 a, 48 b, 52 a, 52 b, and 50 arelocated as close to the largest bubble 24 as possible while retaining abuffer of typically 4 inches (about 10 cm) of measurement range forcontrol purposes. With this geometry, the longest path length forinterference is around 100 inches (250 cm) and at the speed of sound ofabout 13,633 inches per second (about 346 m/sec), this translates into atime lag of 7.3 milliseconds between the transmitted sound pulse 84 andthe worst case stray return signal 88. The transmission of the followingsimultaneous sound pulses 84 a must occur after the worst case soundpulse 84 arrives and preferentially occurs every 10 millisecondsresulting in 100 samples per second for each sensor 48 a, 48 b, 52 a, 52b and 50. If the bubble 24 size is smaller, the sampling rate can beincreased accordingly, depending on the worst case interference pathlength and can be as high as 300 samples per second for layflat film 34that is about 24 inches (about 60 cm).

[0046] According to an embodiment of the present invention, FIG. 8 showsa schematic cross-sectional view of a valve system. This is generallysimilar to a previously used, prior art valve system with two keychanges. First, rather than using an air cylinder as in a prior design,a linear servo 90 is directly connected to air actuating piston 92 fordriving piston 92. Air actuating piston 92 is cylindrical andconcentrically located inside of valve body wall 93. The secondmodification is that a small air gap 94, just large enough to ensure nocontact between air actuating piston 92 and valve body wall 93, isprovided to create frictionless movement of the piston relative to thevalve body wall.

[0047] Air actuating piston 92 moves linearly inside of valve body wall93 to cover and uncover longitudinal air regulating slots 98 to avariable degree determined by the present position of air actuatingpiston 92 under the control of linear servo 90. To ensure no leakageflow of air, valve body end 95 fully covers one end of valve body wall93 with the opposite end remaining open and provides a convenientmounting point for linear servo 90. Piston end 99 partially covers oneend of air actuating piston 92 with the opposite end remaining uncoveredand provides a connection point to linear servo shaft 91. Pressureequalizing holes 96 are provided through piston end 99 to preventpressure or vacuum from building in air volume 97 due to motion of airactuating piston 92 within valve body wall 93 and valve body end 95.

[0048] Valve body flange 89 extends radially outwardly from an open endof valve body wall 93 and concentrically seals it inside of externalpipe 101 forcing incoming air flow 100 to flow fully into the open endof valve body wall 93. As air actuating piston 92 varies the open areaof longitudinal air regulating slots 98, slot air flow 102 is controlledand flows through valve body wall 93 and is contained by thecontinuation of external pipe 101, forcing controlled air flow 104 tocontinue through external pipe 101 to its destination. Except for anyminor leakage in pipes and/or joints or pressure differences due tocontrol, incoming air flow 100, slot air flow 102 and controlled airflow 104 are substantially the same. Further, the positioning responseof 5 to 10 msec to a resolution of 0.004 inch (about 0.1 mm) of linearservo 90, linear servo shaft 91 and air actuating piston 92 allow foraccurate and fast control of controlled air flow 104 allowing forcontrol of natural tube size changes in bubble 24.

[0049] As indicated above, the use of covered and uncovered longitudinalslots was known, but use of a servo motor and functionless movementimprove response time.

[0050] Having described certain embodiments, it should be understoodthat modifications can be made without departing from the spirit andscope of the invention as defined in the appended claims. Furthermore,while certain components may be described as having certain advantages,modifications could be made with other components with some features andadvantages that are different.

What is claimed is:
 1. In an extrusion system having a die for receivingmolten plastic and for providing from the die a bubble that exits thedie in molten form and that solidifies above a frost line, the systemblowing cooling air on the bubble as it exits the die, the systemfurther comprising: a first sensor for sensing a position of the bubbleabove the frost line after the bubble has solidified; a second sensorfor sensing a position of the bubble at a vertical location between apoint just above the frost line and a point below the frost line; acontroller responsive to signals from the first and second sensors forcontrolling the flow of cooling air on the bubble.
 2. The system ofclaim 1, wherein the second sensor is at the vertical location, thesystem further comprising a third sensor at the vertical location, andpositioned approximately such that a line between the first and thirdsensors passes through a diameter of the bubble.
 3. The system of claim2, wherein there are two and only two sensors at the vertical location.4. The system of claim 1, further comprising a third sensor locatedabove the frost line at the same vertical position as the first sensor,and positioned approximately such that a line between the first andthird sensors passes through a diameter of the bubble.
 5. The system ofclaim 2, wherein there are two and only two sensors at the vertical. 6.The system of claim 4, further comprising a fourth sensor at thevertical location, and positioned approximately such that a line betweenthe second and fourth sensors passes through a diameter of the bubble.7. The system of claim 6, wherein there are two and only two sensors atthe same vertical position above the frost line where the bubble hassolidified, and two and only two at the vertical location just above tobelow the frost line.
 8. The system of claim 7, further comprising afifth sensor at a vertical location different from the verticallocations of the first, second, third, and fourth sensors, thecontroller responsive also to the fifth sensor.
 9. The system of claim1, further comprising an air flow valve that governs flow of coolingair, wherein the controlling includes altering flow of air from a supplyblower by altering a position of the air flow valve.
 10. The system ofclaim 9, wherein the air flow valve has openings and a piston, thepiston for covering and uncovering the holes in response to thecontroller.
 11. The system of claim 10, wherein the piston is driven bya servo motor.
 12. The system of claim 10, wherein the piston is spacedfrom a valve body wall around the piston for frictionless movement. 13.The system of claim 1, wherein the sensors are non-contact sensors. 14.The system of claim 13, wherein the sensors emit and sense acousticenergy.
 15. The system of claim 1, wherein the second sensor is justabove the frost line.
 16. The system of claim 1, wherein the secondsensor is at the frost line.
 17. The system of claim 1, wherein thesecond sensor is before the frost line.
 18. The system of claim 1,wherein the controller continuously calibrates the second sensor to asize of the bubble as sensed by the first sensor.
 19. The system ofclaim 1, wherein the controller continuously monitors the first andsecond sensors for errors by comparing the signals to an expected rangeof values.
 20. The system of claim 19, wherein the controller isresponsive to out-of-range data from one of the sensors by locking in aprior good sample.
 21. The system of claim 20, wherein the controllerdisregards data from a sensor that provides a sufficiently long seriesof out-of-range samples, the controller using data from the remainingsensors, and providing a notification that data from the particularsensor is being disregarded.
 22. The system of claim 7, wherein thefirst to fourth sensors emit and sense acoustic energy pulses, whereinthe first to fourth sensors emit pulses simultaneously.
 23. The systemof claim 1, wherein the system monitors the location of the frost lineand moves the second sensor to adjust for changes in the location of thefrost line.
 24. In an extrusion system having a die for receiving moltenplastic and for providing from the die a bubble that exits the die inmolten form and that solidifies above a frost line, the system blowingcooling air on the bubble as it exits the die, the system comprising: aplurality of sensors arranged for monitoring a position of the bubble; acontroller responsive to data from the sensors for controlling a flow ofair to the bubble below the frost line, the controller being responsiveto out-of-range data from one of the sensors for disregarding theout-of-range data from that sensor, for using data from the remainingsensors, and for providing a notification that data from the sensor isbeing disregarded.
 25. The system of claim 24, wherein the notificationincludes causing a warning light to change states.
 26. The system ofclaim 24, wherein data is disregarded after a number of out-of-rangedata samples exceeds a threshold, and is used after a number of datasamples are in the desired range.
 27. In an extrusion system having adie for receiving molten plastic and for providing from the die a bubblethat exits the die in molten form and that solidifies above a frostline, the system blowing cooling air on the bubble as it exits the die,the system comprising two and only two sensors at one vertical heightfor emitting and sensing acoustic pulses to sense a position of thebubble and arranged on opposite sides of the bubble such that a signalemitted by one sensor will be detected by that sensor before it could bedetected by the other sensor.
 28. The system of claim 27, furthercomprising at least one other sensor at a different vertical height. 29.In an extrusion system having a die for receiving molten plastic and forproviding from the die a bubble that exits the die in molten form andthat solidifies above a frost line, the system having at least onesensor for providing a signal based on a position of the bubble, acontroller responsive to the sensor, and an air blowing system forblowing cooling air on the bubble as it exits the die in response to thesignal from the sensor, the system comprising: a flow valve includinglongitudinal slots; a piston movable for covering and uncovering theslots; and a servo motor responsive to the controller for moving thepiston.
 30. The system of claim 29, wherein the piston is surrounded bya valve body wall and spaced from the wall by a gap to provide fornon-contracting movement between the piston and valve body wall.